Scanning devices usually utilize an image sensor to collect light reflected from an object to generate an image of the object. The image sensor converts the light energy into an electrical charge and finally to a set of bits representing the color and intensity of the light. The resulting digital image is for example a two-dimensional image of a relatively flat object such as paper, which is, for example, placed on a flat bed scanner.
3D scanning is a technique of converting the geometrical shape or form of an object into a data set of points. Each point may represent an actual point, in three-dimensional space, of the surface of the object that was scanned. The collected data can be used to construct digital, three-dimensional models. The collected 3D data is useful for a wide variety of applications. For example, the 3D scanning technology is extensively used by the entertainment industry in the production of movies and videogames. Other common applications of this technology include industrial design, orthotics and prosthetics, reverse engineering and prototyping, quality control/inspection and documentation of cultural artifacts.
In particular, 3D data of human faces can be collected. The captured facial contour and geometry can be used to design masks, which are applied in many medical fields including CPAP (constant positive air pressure). The exact 3D modeling of the facial contour eases the manufacturing of well fitting masks avoiding the problem of air leaks, which can cause eye irritations.
There are many different technologies that can be used to build a 3D scanning device. One well known classification divides the scanning technology into two types: contact and non-contact 3D scanning.
Contact 3D scanners probe the object through physical touch, while the object is in contact with or resting on a precision flat surface plate. This type of scanning technology is mostly used in manufacturing and can be very precise. The disadvantage of contact scanning is that it requires contact with the object being scanned. Thus, the object might be modified or damaged during the scanning process. Moreover, this scanning technology is usually very slow compared to other scanning methods. In addition, it is not desirable to use such a 3D contact scanner for scanning the facial contour of a human face.
Another popular class of 3D scanning techniques is active non-contact scanning. Active means that the respective scanner emits some kind of encoded, structured or non-encoded energy, such as light, and detects the reflection of said energy at the scanned object in order to probe the object and to capture a corresponding 3D representation.
Active non-contact type 3D scanners are most frequently based on the use of triangulation. In this case, the scanner for example directs a laser beam on the object and uses a camera to look for the location of the laser dot. Depending on how far away the laser strikes a surface of the object, the laser dot appears at different places in the camera's field of view. The underlying technology is called triangulation because the laser dot, the camera and the laser emitter form a triangle.
Instead of a single laser dot, a pattern of light can be projected on the object. This type of scanner is called structured light scanner and can be regarded as an active non-contact scanner. The pattern may be a one-dimensional pattern like a line or a two-dimensional pattern like a grid or a line stripe pattern. The light pattern is projected onto the object using for example an LCD projector. A camera, offset slightly from the pattern projector, captures the structured light as seen from a camera viewpoint. Further, the captured image is analyzed and the pattern distortions are evaluated. By means of an algorithm based on the triangulation, the distance to each point of the pattern is calculated. By sweeping the light pattern across the surface of the object, a 3D representation of the object can be achieved. Structured light 3D scanners provide very fast scans. Instead of scanning one point or line at a time, structured light scanners can scan multiple points or lines or the entire field of view at once.
However, in order to acquire proper scans, the scanner should be located at a certain distance and/or angle from the object. For a proper alignment of the scanner it is well-known, for example, to use a static aim pattern, which contains markers that need to be aligned with face features by moving and rotating the hand-held scanner. As an example, the eyes of the patient's face need to be aligned with projected circles, while a cross needs to be aligned with the nose tip. This alignment procedures, however, only provides quite inaccurate results and has been found not intuitive enough for sleep technicians that use such scanners. Additionally, if the object and the scanner can freely move (for example scanning of a person's head without chin support by means of a handheld scanner), the user might have difficulties in evaluating the proper scan position. Therefore, some form of feedback or assistance is required for correctly positioning the scanning device with respect to the object to be scanned.